Ion gauges

ABSTRACT

An ion gauge in which the collector is arranged outside the ionisation region and the ions representing the gas pressure are extracted from the ionisation region via an aperture in an X-ray screen and directed by a reflector electrode onto the collector electrode. The ion reflector electrode surrounding the collector electrode is constructed and shaped so that soft X-rays passing through the ion-exit aperture undergo multiple reflections at the reflector electrode before they are able to be reflected onto the collector. A cylindrical reflector electrode is disclosed provided with an internal truncated conical X-ray reflecting member which intercepts soft X-rays and reflects them back and forth several times between the cylindrical and conical reflecting surfaces. The end of the cylindrical reflecting electrode nearest the ion-exit aperture is tapered inwardly to reflect onto the conical reflector X-rays falling on that part of the surface.

United States Patent 1 Pittaway Sept. 16, 1975 ION GAUGES 3,509,418 41970 Oostrom 324 33 3,619,684 11/1971 Andrew et al.. 324/33 [75]Inventor: Lawrence Graham f 3,742,343 6/1973 Pittaway 324 33 Salfords,Near Redhlll, England 1 Assigneei Philips Corporation, New PrimaryExaminerJames W. Lawrence York, Assistant ExaminerB. C Anderson [22]Filed: May 25! 1973 Attorney, Agent, or FirmFrank R. Trifari; Ronald L.

[30] Foreign Application Priority Data May 26, 1972 United Kingdom25017/72 [52] US. Cl. 250/489; 250/505; 317/7 [51] Int. Cl. HOlj 3/108[58] Field of Search 250/489, 503, 508, 515, 250/505; 313/7; 324/33 [56]References Cited UNITED STATES PATENTS 2,759,106 8/1956 Wolter 250/5052,766,385 10/1956 Herrnring et a1. 250/505 2,819,404 1/1958 Hermring eta1. 250/505 3,071,704 l/l963 Reich 313/7 3,109,115 10/1963 Lafferty313/7 3,292,078 12/1966 Herzog 313/7 3,320,455 5/1967 Whetten et a1.313/7 3,341,727 9/1967 Schuemann 313/7 3,463,956 8/1969 Groszkowski324/33 3,465,189 9/1969 Redhead 313/7 3,466,484 9/1969 Clay et a] 313/7Drumheller ABSTRACT An ion gauge in which the collector is arrangedoutside the ionisation region and the ions representing the gas pressureare extracted from the ionisation region via an aperture in an X-rayscreen and directed by a reflector electrode onto the collectorelectrode. The ion reflector electrode surrounding the collectorelectrode is constructed and shaped so that soft X-rays passing throughthe ion-exit aperture undergo multiple reflections at the reflectorelectrode before they are able to be reflected onto the collector. Acylindrical reflector electrode is disclosed provided with an internaltruncated conical X-ray reflecting member which intercepts soft X-raysand reflects them back and forth several times between the cylindricaland conical reflecting surfaces. The end of the cylindrical reflectingelectrode nearest the ion-exit aperture is tapered inwardly to reflectonto the conical reflector X-rays falling on that part of the surface.

8 Claims, 3 Drawing Figures PATENTED SEP 1 6 I975 SHEET 1 BF 3PATENTEBSEP I6 5975 sum 2 or 3 PATENTEU SEP I 6 I975 snmam s ION GAUGESThis invention relates to ionisation vacuum gauges of the kind employingan external collector electrode.

Low gas pressures in a region of space are frequently measured byionisation methods in which electrons emitted by a cathode areaccelerated so that they pass through a mesh wall into an ionisationspace bounded by said mesh wall and located in the region, withsufficient energy to ionise gas present therein. Ions so formed aredrawn by an electrostatic field towards a suitably biassed collectorelectrode and the collector current forms a measure of the amount of gaspresent in the ionisation space and hence of the gas pressure in thesaid region of space. Unfortunately other concurrent phenomena alsocontribute to the collector current and limit the accuracy and range ofgas pressures over which the gauge will operate satisfactorily.

One such phenomena is the generation of soft X-rays from the collisionof electrons with the mesh and other metal structural parts surroundingthe ionisation space, and interception of this X-radiation by thecollector electrode causes the emission of photo-electrons thereby. Asattempts are made to measure lower gas pressures, this photo-electroncurrent eventually swamps the ion measurement current.

Early attempts to reduce the magnitude of the X-ray photocurrent includethe use of a thin wire collector as described by R. T. Bayard and D.Alpert in Review of Scientific Instruments Volume 21 page 571, and theuse of an additional modulator electrode by P. A. Redhead Review ofScientific Instruments Volume 31 page 343. Thus by reducing the surfacearea of the collector electrode the photocurrent was reduced bycomparison with the collected ion current, and by modulating theionisation current component it was made possible to measure a smallerion current in the presence of a given residual current due tophoto-emission and other factors.

A further reduction in X-ray photo-emission from the collector electrodewas achieved by mounting the collector electrode outside the ionisationspace and with drawing ions from the ionisation space via an aperture inan X-ray screen. Examples of such gauges are due to P. A. Redhead in US.Pat. No. 3,465,189 and to J. Groszkowski described and claimed inBritish Pat. No. 1,173,354.

In a development of the external-collector ion gauge, described in US.Pat. No. 3,742,343, the ionisation space is formed within asubstantially equipotential boundary wall by employing a fine aperturemesh which effectively prevents an external electrostatic field frompenetrating the ionisation region and withdrawing ions which wouldotherwise pass to the collector. The apertured X-ray shield ismaintained at the same potential as the mesh, and ions are drawn outtowards the collector by means of a suitably biassed extractorelectrode. The ions are then concentrated onto a collector electrode bymeans of a deflector electrode. However there is a tendency for X-raysto pass through the aperture in the X-ray shield and to be deflected atthe surface of the reflector electrode and thus concentrated onto thecollector electrode.

It is an object of the invention to provide an improved ion gaugeemploying an external collector, in which the residual X-rayphotocurrent can be significantly reduced.

According to the invention there is provided an ion gauge comprising asource of electrons, an enclosure formed of electrically conductingwalls surrounding an ionisation space in which in operation a gas whosepressure is to be measured is present, the Walls of said enclosureincluding a peripheral apertured region form ing an electronaccelerating grid such that in operation electrons are directed thereatby the application to said source mounted outside said enclosure of anegative bias relative to said enclosure sufficient to cause at leastsome of said electrons to pass into the ionisation space and to ionisegas present therein, said enclosure having an end wall substantiallyopaque to the passage of soft X-rays and provided with an ion-exitaperture through which ions formed in said ionisation space by saidelectrons can pass out of said ionisation space, a thin rod-likecollector electrode mounted externally with respect to said ionisationspace and directed along the axis of said ion-exit aperture and biassedin operation to collect ions from said ionisation space via saidion-exit aperture to provide a current dependent on the pressure of gasin said ionisation space, and a deflector electrode arranged to surroundsaid collector electrode and so biassed that said ions are caused toconverge and strike said collector electrode, said deflector electrodebeing so formed that at least the major portion of X-radiation formed bycollision of electrons with the walls of said ionisation space andcapable of striking the collector electrode after passing through saidionexit aperture, is caused to suffer multiple reflections at saiddeflector electrode and thereby to be substantially reduced in intensitybefore being incident on said collector electrode.

The deflector electrode can be formed so that the major portion of theX-radiation passing through the ion exit aperture is caused to bereflected more than three times before reaching the collector electrodeand the X-ray reflecting portions of said deflector electrode preferablyhas an X-ray reflection coefficient of less than 0.5. The deflectorelectrode can include a cylindrical surface and a further surface ofrevolution arranged within said cylindrical surface coaxial both withsaid cylindrical surface and with said collector electrode, so that atleast the major portion of said X- radiation passing through saidaperture and directed onto the inward facing said cylindrical surface,is reflected onto the outward facing said further surface of revolutionand thereafter suffers multiple reflection between said surfaces. Thesaid further surface of revolution can comprise a truncated conicalmember the apex of which is directed towards the ion exit aperture. Theinward facing cylindrical surface can be tapered gradually inwardstowards the end adjacent the ion exit aperture so that Xrays incidentthereon which would in the absence of said taper be reflected onto thecollector electrode, are reflected onto the outward facing conicalsurface instead.

The end wall of the ionisation space having therein the ion-exitaperture, can be electrically connected to the remaining walls of theionisation space so as to be maintained at the same potential as saidwalls and an annular extractor electrode can be arranged adjacent theion exit aperture outside the space enclosed by said walls and biassednegatively with respect to the walls of the ionisation space so as toextract ions from the ionisation space via said ion-exit aperture.

In order that the present invention may be clearly understood andreadily carried into effect an embodiment thereof will now beparticularly described by way of example, with reference to theaccompanying drawings of which:

FIG. 1 is a longitudinal section of an ion gauge embodying theinvention,

FIG. 2 is a longitudinal section of the extractor and deflectorelectrode assembly of a prior form of external collector gaugeillustrating the paths taken by X-rays entering via the ion-exitaperture, and

FIG. 3 is a longitudinal sectional outline diagram of part of the iongauge of FIG. 1 illustrating the paths taken by X-rays entering thecollector region via the ion-exit aperture.

An ion gauge embodying the invention is illustrated in longitudinalsection in FIG. 1 to which reference will now be made. The ion gaugeshown in FIG. 1 is normally called a nude ion gauge since it is notprovided with an airtight surrounding envelope and is intended to bemounted in apparatus, within which the pressure is to be measured, bymeans of a flange 23.

Cathodes 1, 2 in the form of electron emissive filaments are connectedto support and connection wires 21 mounted in glass seals 25, and whichproject therefrom so that heating and biassing potentials can be fedthereto. It is desirable to employ filaments which can be operated at arelatively low temperature, for example a rhenium filament coated withlanthanum hexaboride, to prevent dissociation of hydrogen present,however any convenient filament and coating material can be employed.

The glass seals 25, which form insulating supports for variouselectrodes via respective support wires, are attached to and each form aseal with a metal plate 26 which latter is sealed to a conductingcylinder 24. The metal flange 23, welded to the cylinder 24, enables theion gauge to be attached to a vacuum system to form a seal therewith.The glass employed for the seals 25 is chosen to have a coefficient ofthermal expansion which is compatible with that of the plate 26.

An ionisation space 13 is bounded by a conductive electron accelerationgrid assembly 3 comprising a peripheral wall 4 and one end wall 5, bothformed of a closely woven wire mesh, conveniently of Tungsten, and afurther end wall 6, substantially opaque to soft X-rays, and formed ofthin sheet metal, conveniently stainless steel. The mesh size of theclosely woven wire mesh forming the walls 4 and 5 is such that thelargest dimension of each aperture in the mesh is small enough toprevent the field present, in operation, outside the electronaccelerating grid 3 due to a potential difference applied between thegrid 3 and the cathodes 1, 2 and between the grid 3 and the surroundingswhich comprise principally the grounded electrically conductingcylindrical screen 24, conveniently of stainless steel, from penetratingthe ionisation space 13 to a significant extent. In this way conditionscan be provided in normal operation so that, when an electronacceleration field is employed which is sufficient to provide optimumionisation of the gas whose pressure is to be measured, the collectoroutput current does not fall significantly when the electronaccelerating field is in creased while other parameters, such as gaspressure and other electrode potentials, are maintained constant. Otherforms of apertured conducting material can alternatively be employed forthe electron transparent wall region 4, 5, for example a suitable metalsuch as copper, electrodeposited on a mesh base and then stripped fromthe base, provided that the largest dimension of each of the aperturesis small enough to prevent the electrostatic field set up outside thegrid 3 from significantly penetrating the ionisation space 13.

The end wall 6 of the electron accelerator grid 3 is provided with anoutwardly directed truncated conical portion 20 surrounding an ion-exitaperture 7. A generally cylindrical deflector electrode 31, formedconveniently of stainless steel or alternatively of molybdenum,tungsten, tantalium or other conducting material having a relatively lowX-ray coefficient of reflection, and having at one end a wall 32, isconductively attached, for example by welding, to a connection andsupport rod 34 by which means the deflector electrode 31 can either beconnected to the electron accelerator grid 3 or to another suitablesource of ion repelling potential.

An annular extractor electrode 39, formed conveniently of stainlesssteel and having a V-shape crosssection, is mounted between the conicalsurround 20 of the ion-exit aperture 7 and the open end portion 33 ofthe deflector electrode 31 but is insulated therefrom and is providedwith a conducting support 30 which forms the connection to a source ofion extracting bias potential, not shown. The inner radial extremity ofthe annular extractor electrode 39 projects inwardly, in a radial sense,slightly beyond the edge of the aperture 7 and the edge of the open.endportion 33 towards the axis of the path taken by ions extracted via theaperture 7. The outer diameter of the electrode 39 is greater than thatof the surrounding edge of the aperture 7 and is therefore partlyscreened by the end wall 6 from ions within the ionisation space.

A collector electrode 10 in the form of a wire, conveniently a tungstenwire extends along the axis of the extracted ion path within thedeflector 31 and is mounted on a conductive support member 44, whichextends through an aperture in the wall 32 and through a seal 25 to forma terminal connection for the collector elec trode.

The deflector electrode 31 includes a hollow truncated conical member 35electrically connected thereto and arranged so that the collectorelectrode projects through the aperture formed by the truncated apex fora short distance. The member 35 can be made of the same substance as theremainder of the deflector electrode 31 and should exhibit a relativelylow reflection coefficient to X-rays directed at the outer surfacethereof.

The open end 33 of the cylindrical portion of the deflector electrode 31is preferably formed so that the inward facing cylindrical surface istapered gradually inwards towards the end adjacent the extractorelectrode 39.

A bead 36 of an insulating substance, such as glass or ceramic, isprovided on the collector electrode 10 and arranged to screen the end ofthe collector support 44 from X-rays directed thereat from the walls ofthe ionisation region 13 via the ion-exit aperture 7. That part of thecollector support 44 situated between the end wall 32 of the deflector31 and the seal 25, is shielded from the incidence of charged particlespresent outside the extraction reflection system by a surroundingconducting cylindrical shield 37 attached and connected to the end wall32.

In operation the cathodes l, 2 are biassed negatively with respect tothe grid 3 so that electrons are accelerated towards the mesh wall 4 andpenetrate the apertures therein with sufficient energy to ionise gaspresent in the space 13. The screen 24 is connected to ground and thecathodes 1, 2 are biassed positively with respect thereto. The extractorelectrode 39 and the collector are biassed negatively with respect tothe cathodes 1 or 2. The deflector 31 is biassed positively with respectto the collector 10, to a potential equal to or greater than that of thegrid 3. Electrons present within the grid 3 form a space charge which,together with the screening effect of the closely woven mesh walls 4 and5, produce an electric field within the ionising region 13 which tendsto prevent positive ions formed therein from escaping via apertures inthe walls 4 and 5 and to cause the positive ions to tend to move towardsthe central part of the ionisation region.

The potential on the extractor 39 sets up an ion extraction field whichdraws positive ions out of the ionisation region 13 via the aperture 7towards the collector 10. The converging effect of the electron lensformed between the extractor 39 and the conical portion of the wall 6causes the ions to follow paths which, in general, avoid collision withthe extractor 39. The further converging lens formed between theextractor 39 and the deflector 31 further assists in directing ions awayfrom the extractor 39 and towards the collector electrode 10.

The collector electrode is so situated that soft X- rays generated bythe collision of electrons with the peripheral grid portion 4 of theacceleration grid 3 are substantially shielded therefrom by the end wall6. However, soft X-rays generated at the end wall 5 and in that part ofthe peripheral grid 4 adjacent thereto, can pass through the ion-exitaperture 7, strike the inside surface of the cylindrical portion of thedeflector electrode 31, and be reflected therefrom.

In the prior form of external-collector ion gauge described in copendingBritish patent application No. 53007/69 (PHB 32008), the soft X-rayswhich pass through the ion-exit aperture and strike the insidecylindrical surface of the deflector electrode, tend to be reflected andconcentrated onto the collector electrode because of the rotationalsymmetry about the axis of the extracted ion beam, and the end wall ofthe deflector electrode.

FIG. 2 illustrates in longitudinal section typical paths taken by softX-rays passing through the ion-exit aperture 7 from the walls of theionisation space 13 into an ion extractor assembly similar to thatillustrated in FIG. 5 of the aforesaid British patent application No.53007/69 (PHB 32008). Two ray paths are shown. X- rays following thepath 46 will be reflected by the cylindrical inner surface of thedeflector electrode 51 and, because of the rotational symmetry, willtend to be concentrated onto the surface of the collector electrode 50where, of course, the incident X-rays will cause the photoemission ofelectrons and hence add an undesired component to the collector currentwhich is not pressure dependent and will tend to swamp thepressuredependent ion current to be measured at lower pressures. Anothertypical X-ray path is indicated by the reference 47, and this undergoestwo reflections, at the inner cylindrical surface of the deflector 51and at the end wall 52, before reaching the collector electrode.

Most metals have a relatively low reflection coefficient, in the regionof from 0.2 to 0.3 for polished surfaces of for example molybdenumtungsten or tantalum, but since a large proportion of the soft X-rayspassing via the aperture can readily reach the collector electrode inthe arrangement shown in FIG. 2 after only one or two reflections, theresidual intensity of the X- rays incident on the collector electrodecan cause a significant undesired residual collector current component.

Referring now to FIG. 3, which similarly illustrates typical paths takenby soft X-rays passing through the ion-exit aperture 7 from the walls ofthe ionisation space 13 into the ion extractor assembly shown in FIG. 1and embodying the invention, it will be seen that the inner surface ofthe cylindrical deflector 31 and the outer surface of the truncatedconical member 35 cooperate to cause the possible X-ray paths to undergoat least three and preferably more than three reflections beforereaching the collector electrode. Thus the rays 55 and 56 suffer aplurality of reflections between the cylindrical inner surface of thedeflector 31 and the outer conical surface of the member 35 before reaching the surface of the collector electrode 10. Since the reflectioncoefficient for each reflection is small, the intensity of the reflectedbeam will decline rapidly as the number of reflections suffered by theX-radiation increases.

The deflector 31 can take the cylindrical form shown in FIG. 2, and theopen end is indicated by the dotted profile 33' in FIG. 3. However,X-rays entering at an oblique angle via the aperture 7, such as the ray57, would tend to be reflected onto the collector electrode 10 afteronly one reflection, as the ray 57. In the preferred embodiment,therefore, the inward facing cylindrical surface of the deflectorelectrode 31 is tapered gradually inwards over the region 33 towards theend adjacent the extractor electrode 39 and the ion-exit aperture 7. Itcan be seen that, as a result, the reflected beam path 57" is directedonto the outward facing surface of the conical member 35 and isrepeatedly reflected between the two surfaces a plurality of times,after which the resultant intensity is negligible.

The extractor electrode 39 is preferably shaped so that the tip 41 ofthe V-shaped cross-section has as small a diameter as possible. When theextractor electrode is shaped with a near-cylindrical portion at thetip, as shown at 40 in FIG. 2, there will be presented a relativelylarge area from which X-radiation can be reflected directly onto thecollector electrode in a manner similar to that of the ray 57' in FIG.3.

It should be noted that the inner surface of the deflector electrode 31including the portion 33, and the outer surface of the truncated conicalmember 35 should be polished to ensure reflection of X-radiation in thedesired direction. An unpolished and therefore diffusing surface wouldgive uncontrolled scattering and would increase the chance that asignificant amount of X-radiation would reach the collector after onlyone or two reflections.

As is well known, the best sensitivity for low pressure measurement canbe obtained by employing a modulator electrode, and this can beintroduced into the ionisation space 13 via an aperture in the end wall5 of the grid 3. It has been found by experiment that a gauge embodyingthe invention can enable the lower limit of pressure measurement to beextended by a factor of 5.

I claim: 1. An ion gauge having a low x-ray photo-electron current,comprising:

a source of electrons;

a conductive electron accelerating grid cage adjacent said source whichcage may be suitably biased so as to accelerate electrons from saidsource into said cage to collide with and ionize gas molecules therein,said cage having a conductive wall substantially opaque to any x-raysgenerated by collision of said accelerated electrons with said cage,said wall having an aperture through which gas ions may be extractedfrom the interior of said cage, the mesh of said cage being sufficientlysmall so as to shield the interior of said cage from electronaccelerating fields exterior thereto;

an annular extractor electrode exterior to said cage and adjacent andsurrounding said aperture but insulated therefrom, which extractorelectrode may be suitably biased to draw gas ions from said cage;

a thin rod-like collector electrode mounted external to said cage withthe axis thereof directed toward said aperture, which collectorelectrode may be suitably biased to collect gas ions drawn from saidcage by said extractor electrode; and

hollow deflector electrode positioned adjacent to but insulated fromsaid extractor electrode for receiving gas ions drawn from said cage bysaid extractor electrode, said deflector electrode surrounding saidcollector electrode so as to deflect 30 gas ions toward said collectorelectrode upon suitable biasing of said deflector electrode, saiddeflector electrode having at least two interior surfaces which convergeaway from said aperture to form a relection trap for soft x-rays, theinterior surfaces of said deflector electrode being so angled that mostsoft x-rays coming from said cage through said aperture and strikingsaid deflector electrode suffer multiple reflections in said reflectiontrap before striking said collector electrode, thereby substantiallyreducing the intensity of soft x-rays striking said collector electrodeand the photo-electron current generated thereby.

2. An ion gauge as defined in claim 1 wherein said deflector electrodeincludes a cylindrical inner surface and an inner surface in the shapeof a truncated cone, said cylindrical and truncated cone surfaces bothbeing coaxial with said collector electrode opposing one another andconverging away from said aperture to form a reflection trap.

3. An ion gauge as defined in claim 2 wherein said collector electrodeprotrudes from the apical end of said truncated cone surface and saidcylindrical surface surrounds said cone surface.

4. An ion gauge as defined in claim 3 wherein said cylindrical surfacehas a larger diameter than said aperture and at the end thereof adjacentsaid aperture tapers inward to a diameter substantially the same as saidaperture.

5. An ion gauge as defined in claim 1 wherein the soft x-ray reflectingsurfaces of said deflector electrode have an x-ray reflectioncoefficient of less than 0.5.

6. An ion gauge as defined in claim 1 wherein said annular extractorelectrode has an inside diameter smaller than said aperture and smallerthan the diameter of said hollow deflector electrode adjacent thereto.

7. An ion gauge as defined in claim 6 wherein said annular extractorelectrode has a v-shaped cross-section with the apex thereof directedinward.

8. An ion gauge as defined in claim 1 wherein said wall has a truncatedconical region surrounding said aperture directed away from said cage.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. I 3 90 23'7 DATED September 16, 1975 |NVENTOR(5) LAWRENCEGRAHAM PITTAWAY It is certified that error appears in theab0ve-identified patent and that said Letters Patent are herebycorrected as shown below:

IN THE SPEC IFICAT ION Column 1, line 61, chance "deflected" to-reflected;

line 62,, change "reflector" to -deflector-,-

Column 4, line 65, change "reflection" to -deflection.

Signed and Scaled this A ttes t:

RUTH C. MASON C. MARSH Arresting Off ALL DANN ummr'ssium'r oj'latenlsand Trademarks

1. An ion gauge having a low x-ray photo-electron current, comprising: asource of electrons; a conductive electron accelerating grid cageadjacent said source which cage may be suitably biased so as toaccelerate electrons from said source into said cage to collide with andionize gas molecules therein, said cage having a conductive wallsubstantially opaque to any x-rays generated by collision of saidaccelerated electrons with said cage, said wall having an aperturethrough which gas ions may be extracted from the interior of said cage,the mesh of said cage being sufficiently small so as to shield theinterior of said cage from electron accelerating fields exteriorthereto; an annular extractor electrode exterior to said cage andadjacent and surrounding said aperture but insulated therefrom, whichextractor electrode may be suitably biased to draw gas ions from saidcage; a thin rod-like collector electrode mounted external to said cagewith the axis thereof directed toward said aperture, which collectorelectrode may be suitably biased to collect gas ions drawn from saidcage by said extractor electrode; and a hollow deflector electrodepositioned adjacent to but insulated from said extractor electrode forreceiving gas ions drawn from said cage by said extractor electrode,said deflector electrode surrounding said collector electrode so as todeflect gas ions toward said collector electrode upon suitable biasingof said deflector electrode, said deflector electrode having at leasttwo interior surfaces which converge away from said aperture to form arelection trap for soft xrays, the interior surfaces of said deflectorelectrode being so angled that most soft x-rays coming from said cagethrough said aperture and striking said deflector electrode suffermultiple reflections in said reflection trap before striking saidcollector electrode, thereby substantially reducing the intensity ofsoft x-rays striking said collector electrode and the photo-electroncurrent generated thereby.
 2. An ion gauge as defined in claim 1 whereinsaid deflector electrode includes a cylindrical inner surface and aninner surface in the shape of a truncated cone, said cylindrical andtruncated cone surfaces both being coaxial with said collector electrodeopposing one another and converging away from said aperture to form areflection trap.
 3. An ion gauge as defined in claim 2 wherein saidcollector electrode protrudes from the apical end of said truncated conesurface and said cylindrical surface surrounds said cone surface.
 4. Anion gauge as defined in claim 3 wherein said cylindrical surface has alarger diameter than said aperture and at the end thereof adjacent saidaperture tapers inward to a diameter substantially the same as saidaperture.
 5. An ion gauge as defined in claim 1 wherein the soft x-rayreflecting surfaces of said deflector electrode have an x-ray reflectioncoefficient of less than 0.5.
 6. An ion gauge as defined in claim 1wherein said annular extractor electrode has an inside diameter smallerthan said aperture and smaller than the diameter of said hollowdeflector electrode adjacent thereto.
 7. An ion gauge as defined inclaim 6 wherein said annular extractor electrode has a v-shapedcross-section with the apex thereof directed inward.
 8. An ion gauge asdefined in claim 1 wherein said wall has a truncated conical regionsurrounding said aperture directed away from said cage.